The present invention relates generally to gear pumps and, more particularly, to a novel improved means for self-lubricating the gear shafts of the pump utilizing the material being pumped, typically a molten viscous thermoplastic material.
Gear pumps are commonly utilized in thermoplastic extrusion systems and other similar applications to provide a positive displacement means of pressurized metered conveyance of molten thermoplastic material. One common application is to interpose a gear pump between the output end of a thermoplastic extruder and the input to an extrusion die to provide a constant volumetric rate of material pumping through the die. It is not uncommon for the meshing gears in such pumps to be driven at significant rotational speeds, whereby proper lubrication of the gear-supporting shafts in the gear pump housing is critical to maximizing the service life of the gear pump. Perhaps the most common means of lubricating the gear shafts is to provide bearings which support the opposite ends of each gear shaft and to form such bearings with a system of channels or grooves to permit a small quantity of the thermoplastic material being pumped to progressively enter the annular space between each shaft and its supposing bearing. The molten viscous character of the thermoplastic material provides a quite suitable form of lubrication and, advantageously, avoids the necessity of providing any independent means of lubrication. Various examples of such self-lubricating gear pumps are found in the prior art, such as U.S. Pat. Nos. 2,276,107; 2,816,511; 2,756,684; 2,471,149; 3,447,472; 4,090,820; 4,160,630; 4,265,602; 4,395,207; 4,470,776; 4,629,405; 4,859,161; 4,927,343; and 5,120,206.
One problem which is sometimes encountered in such self-lubricating gear pumps is premature failure of the pump due to excessive wearing of the gear shafts. While such failures are relatively rare in many thermoplastic pumping applications, it has been discovered that a higher incidence of such pump failures occurs when the material being pumped is a so-called "filled" material, i.e., having a process additive suspended in the thermoplastic polymer. For example, it is quite common in the thermoplastic extrusion of polyester to suspend a small proportion of antimony in the polymer, typically no more than about one percent by weight. Upon inspection of gear pumps which have failed in polyester extrusion applications, it has recently been observed that a significant concentration of antimony had collected in the lubrication grooves formed in the pump bearings. The gear shafts in such pumps also exhibited ring-like wear points annularly about the shafts. It therefore has been theorized that a significant contributing cause to the failure of such pumps has been a tendency of the antimony additive to adhere to the surfaces within the lubrication channels.